1. Field of the Invention
The present invention relates to a membrane switch for various input devices such as a touch panel coordinates input device.
2. Description of the Prior Art
The membrane switch has pairs of contacts each formed on the opposite surfaces of upper and lower sheets placed one over the other so that each pair of contacts are brought into contact with each other by pressing the upper sheet at a position corresponding to the pair of contacts. Such a membrane switch has been applied to the touch panel coordinates input device.
Referring to FIGS. 4 and 5, a conventional membrane switch generally comprises a switching unit 4 having an upper sheet 1 and a lower sheet 2 each formed of a flexible transparent synthetic resin sheet, such as a transparent polyester sheet, and placed one over the other with a spacer 3, such as a dot spacer, therebetween, and an output flat cable 5 through which signals are derived. An upper contact 6 is formed in a functional surface of the upper sheet 1 by etching an ITO (In.sub.2 O.sub.3, SnO.sub.2) film formed over the functional surface by an evaporation process in a desired pattern. Lower contacts 7 are formed likewise in a functional surface of the lower sheet 2. The upper sheet 1 is placed over the lower sheet 2 with the functional surface thereof carrying the upper contact 6 facing the functional surface of the lower sheet 2 carrying the lower contacts 7. The upper contact 6 is formed substantially over the entire area of the functional surface of the upper sheet 1. The lower contacts 7 are formed in a plurality of sections in the functional surface of the lower sheet 2. Normally, the upper contact 6 and the corresponding lower contacts 7 are spaced apart by the spacer 3. When the upper sheet 1 is pressed with a finger tip or a tip of a stick, the upper sheet 1 is caused to flex, so that the upper contact 6 is brought into contact with the corresponding lower contact 7 for switching operation. Terminals 8 and 9 are formed respectively in the respective functional surfaces of the upper sheet 1 and the lower sheet 2 so as to be connected respectively to the upper contact 6 and the lower contacts 7. The terminals 8 and 9 are connected respectively to leader lines 10 and 11. The output flat cable 5 is formed by printing the leader lines 10 and 11 respectively on the opposite surfaces of a base film 12, such as a polyester film. The output flat cable 5 is held between the upper sheet 1 and the lower sheet 2. The upper sheet 1 and the lower sheet 2 are joined adhesively along the respective peripheries thereof with a two-side adhesive tape 13 having a predetermined thickness so that a predetermined gap corresponding to the thickness of the two-side adhesive tape 13 is formed between the upper sheet 1 and the lower sheet 2.
The membrane switch thus formed is attached adhesively with a two-side adhesive tape 15 to a base plate 14, such as an acrylic plate or a glass plate. The base plate 14 carrying the membrane switch is mounted on an apparatus such as a CRT.
In a process for adhesively joining the upper sheet 1 and the lower sheet 2 with the adhesive tape 13, the adhesive tape 13 lined with a separating paper is applied to the upper sheet 1 or the lower sheet 2, then the separating paper is removed from the adhesive tape 13, and then the upper sheet 1 and the lower sheet 2 are joined together. However, removal of the separating paper from the adhesive tape 13 requires a very difficult and troublesome work in assembling the membrane switch increasing the assembling cost. Furthermore, since the adhesive tape 13 is used simply for adhesively joining the upper sheet 1 and the lower sheet 2, the output flat cable 5 needs to be held between the upper sheet 1 and the lower sheet 2 by another adhesive means, which also increases the steps of membrane switch assembling process.